Recently, radiotherapy has been shown to be an effective treatment for the prevention of restenosis following angioplasty. In catheter-based systems for intravascular brachytherapy (IVB) a radioactive source is placed in the lumen at a short distance from the target, typically 1 to 3 mm. Both gamma and beta emitters are currently in use for IVB. All sources produce dose distributions with a steep dose gradient in the target volume with dimensions of a few millimeters. However, high energy photon emitters (e.g. (192)Ir), low energy photon emitters (e.g. (103)Pd) or beta emitters (e.g. ((90)Sr/(90)Y) have very different dosimetric and physical characteristics in terms of depth dose penetration, attenuation effects and shielding requirements. IVB delivery devices based on (192)Ir and (90Sr/90Y) have become commercially available and one based on (103)Pd is under development. There is now a critical need to obtain dosimetry parameters for these systems for the determination of dose distributions in the target and surrounding volumes. There is a need to modify the dose calculation formalism of the AAPM Task Group No. 43, which is suited to point-like interstitial brachytherapy sources. We propose a calculation formalism based on cylindrical coordinates, which is better suited to the geometry of catheter-based P/B sources. Also, there is a critical need to validate the manufacturers' dosimetry data by independent investigations using both dosimetry measurements and calculations. We propose to use two independent dosimetry methods (radiochromic film and thermoluminescent [TL} sheet dosimetry) and Monte Carlo calculations (ITS, MCNP and EGS4 codes) to intercompare various results and obtain an accurate set of dosimetry parameters for IVB systems. Our hypothesis is that successful implementation of the various IYB systems requires a better understanding of dosimetry issues at millimeter distances. The principal objective is to determine the dosimetry parameters in the immediate vicinity (within mm) of the various radionuclides (both photon emitters and beta emitters) and to examine critically the dosimetry issues of importance in the clinical implementation of IVB for prevention of restenosis following angioplasty. The goals of the project are to develop tools for the optimization of dose distributions produced within a target volume and uniformity of dose along and around a blood vessel.